Electrolytic process and apparatus for the production of chromic acid and caustic alkali



May 25, 1937. J. w. BOSS 2,081, 787

v ELECTROLYTIC PROCESS AND APPARATUS FOR THE PRODUCTION OF CHROMIC ACID AND CAUSTIC ALKALI Filed June 15, 1936 Fg'g. 1.

Patented May 25, 1937 ELECTROLYTIC PROCESS AND APPARATUS F OR THE PRODUCTION OF CHBOMIC ACID AND CAUSTIC ALKALI John W. Boss, Livingston, Mont.,'

Chromium Products Corporation,

assignor to Livingston,

Mont., a corporation of Montana Application June 15,1936, Serial No. 85,405

7 Claims. (01. 204-9) The present invention relates to an improved electrolytic process and apparatus for the production of chromic acid and caustic alkali. This invention relates to an improvement in the process 5 described in my co-pending application Serial No. 6904 and an improvement in the apparatus described in my co-p'ending application Serial No. 25,625.

An object of the present invention is to produce in a single cell pure chromic acid from a soluble chromate salt.

A further object is to overcome the defect caused by accumulation of sludge between the feed solution and the caustic alkali solution in 15 cells of this type as heretofore known.

A further object is to revivify used chromium plating solutions. v

Another object is to produce in a single cell both crude and finished chromic acid.

20, Still another object is .to provide means for effecting more complete removal of basic sludge from the cell than has been possible by other means. i

With the foregoing and other objects in view,

' the invention will-be more fully described hereinafter, and will be more particularly pointed out in the claims appended hereto.

In the drawing, wherein like symbols refer to like or corresponding parts throughout the several views' Figure l is a central vertical section of a cell in operating condition, and

Figure 2 is a top plan view of a modification of the cell in which the feed pipes do not lie in 35 the same vertical plane.

This invention relates to a process and apparatus for the electrolytic production of chromic acid and of caustic alkali. In the prior art it is customary to produce chromic acid (CrOz) by 40 the action of an acid such as sulphuric on an alkali chromate or dichromate such as sodium chromate or dichromate. Separation of the chromic acid is accomplished by crystallization and draining off the mother liquor containing 45 in solution other products of the reaction.

Some of the disadvantages of such a process are: That difliculty is encountered in removing other acid radicals from the chromic acid; and that the alkali originally combined as chromate 50 or dichromate appears as a salt such as sodium sulphate. This salt is not of much value and is not suitable for use in preparing new quantities of sodium-"chromate.

When a used" plating solution is. 'revivifled 55 according to the procedure taught in the applimon chamber or compartment I4.

is provided near the upper part of the compartcations above mentioned some diificulty arises because of the precipitation of bases at the plane separating the caustic'soda solution from the impure acid solution. As the acid solution uriderlies the caustic solution these bases 6r sludges tend to settle in the acid, redissolve and reprecipitate at the plane of separation. This cyclic behavior lowers the efficiency of the cell and requires an excessive flow of caustic soda solu- .tion to carry out the precipitates; 'it also requires 10 closer supervision of the cell to regulate the removal of these precipitates. In the present invention as applied to revivifying used plating acid there is no contact between it and the caustic solution. Instead the caustic alkali solution floats on one part of an alkali chromate feed solution while an impure chromic acid solution floats on another part of the feed solution.

The used plating acid is then mixed with pure chromic acid solution and floated on top of the impure chromic acid solution. The alkali chromate feed solution can be maintained neutral or nearly so and bases precipitated at its contact surface with the crude acid solution do not tend to redissolve but settle'out towards the bottom of the feed solution.

In the drawing l0 designates the body of a cell which may be formed of glass, stoneware or other suitable material resistant to the corrosive action of the solutions employed. The cell Ill is divided longitudinally into two equal come partments by a partition H which extends down into the cell a substantial distance but stops short of the bottom of the cell. As shown the partition H extends about three-quarters of the .way down into the cell. There are, therefore,

formed two side compartments I 2 and I3 com-, municating at their lower ends to form a com- The cell In ment l2 withan overflow pipe IS. A movable .sleeve l6 connects the pipe I5 to the take of! IT.

The distance between the take off l1 and the overflow pipe 15 may beregulated by means of the sleeve Hi to alter the rate of overflow from the compartment l2. Alsolower down in the compartment I! but yet a substantial distance above the bottom ofthe partition II the cell W has a second overflow pipe. [8 similarly provided with a sleeve l9 and takexoff llll. In the compartment l3 approximately opposite to the overflow pipe IS in the other compartment, is placed anfoverflow pipe 2|. .with sleeve 22 and take off 23. At one side of the common compartment l4 and below the lower edge of the partition II is lower in the compartment l3 approximately opposite the overflow pipe 2|.

In the following description sodium chromate will be used as an example but it is to be understood that any alkali salt of chromic acid may be used. In filling the, cellthe compartment ll is filledwith a solution of sodium chromate of spe-v cific gravity, for example, 1.450. This solution is introduced through the feed pipe 29 and the solution is added until it fills the lower ends of the side compartments l2 and H. In the com-' partment l3 on top of the sodium chromate.

solution is floated a caustic soda solution of, for example, specific gravity 1.200. This solution i added through the supply pipe 30 to substantially 'fill the compartment l3. In the other side compartment I2 is fioated first a crude chromic acid solution of specific gravity, for example, 1.325. This solutionis added through'the feed pipe 3|. The amount of this solution must be such as, to extend from the top of the sodium chromate solutionto some point below the anode 21., Finally, on top of thisv crude chromic acid solution is floated a 'pure chromic acid solution of specific gravity, for "example; 1.200. This last solution is added through its teed pipe 3'2.

lt' will be'necessary, of course, inljadding these various solutions to balance the heads 0i solu' tions in the opposite chambers so as to prevent mixing beneath the l partition II. It is also necessary, as soonas "the: solutions are added,

that current be passed between the electrodes so as .to prevent difiusion of the' vario'us solutions.

Whenthe electrodes 'are'co'nnected to a' suit-- able source for direct current the pure chromic M ld solution loses basicfions to the'i mpure solution ,on' which it floats while the'crude solution loses chromateions to. the solutions above"it..

The pure solution thus tends tofincrease in spe ems-l ra W e l m s lu n 1 i spe i c ravi y; In view or this factr it' is neces- ,sary to regulate.the speoificjgravity of the crude 'solutionin, order, to, prevent'mixing with the l h ii t; t c n lu de s i s am as nenn si. ."tsoiu io rr .t njsup iiaiihea .Pu 56. mm t e an #01. mf ef 2 specific gravities .due to the operation of the The ,crude solutioni receiv'es' iron the sodium and 1 assis ants, J xelower; co partment basic ions and the on i in the. ifiml aiftment" 3 infi m. ibbth regulating. oilthe ,Eiensi t'y inorder to mainthe plane of ,eparation between it and the fl ro te jsb ut ofi'u d r ri s ,1 chromate feed solution -yo i adug to e s its sil bcr bed the cr de c l onnc "acid bove identified apphcationszf The caustic t me andlm,dnsityrequifinijdrswine or:

and the plane of separation between it and the solutions floating above it moves downward toward the partition II. It is, therefore, necessary to replenish this solution to prevent mixing of the solutions in the other compartments. The sodium chromate solution also tends to accumulate some caustic soda. some basic substances such as aluminum, forming soluble com-..

pounds with the caustic soda, may also accumulate in this feed solution. Inaddition to this there is a sludge of' insoluble basic materials which may appear at the separation plane between the chromate and the crude chromic acid. This" sludge is insoluble in the sodium chromate and eventually settles to the bottom. of the cell where it lies until such time as the cell maybe cleaned.

In operation of this cell it is or the utmost importance to maintain the planes of separation between the various solutions. 'I'hus, i f the sodium chromate feed solution be permitted to drop below the lower edge of the partition I l the solutions in the side compartments will mingle and the contents of the cell destroyed. If, on the other hand, the sodium chromate solution be allowed to rise as high as the anode 2B evolu- 1 tion of gas will cause breaking of the plane o1 tween thefplain andcrude chromic acid solutions must not drop as low as the opening of the feed pipe 3| or the opening of the ioverfi ow 'pipe i8 as this would cause mixing of the two chromic acid solutions. 'Also' the plane between these acid solutions must not rise as high as the anode 21' for here gas evolution would destroy the plane of separation between the solutions.

, In view of the requirement for regulating these planes oi-separation it will be seen to be necessary that some means of observing the planes be'p'ro- 'vide'di Thus the tens themselves may-be made of transparent material, for the various chromate solutions" are readily" distinguishable by.

"color while the caustic soda is "similarly distin-- guishable. by its lack of color. An alternativemeans of observing the planes is to lower a'glass tube into the cell, close the top and carefully 'w ithdrawit from'the cell. The solutions-carried in the glass tube will indicate by their color th cel b tw n these l o s The inost convenient, waytooperam this cell and'the'wayior which the cell illustrated is iiie d v that the ower'endsoi the tubes 30, ll, and 32 are turned s gs to-directt t eamsor incoming theobicebemg tofavoiddisturbin'g this plane.

The feed pipes and ov rnowj pesare V andconne'cted with sources 01 solution to proand planes; of separation the relation'existing in continuous ration. Itv will be'noted I v quidawayiiro'n the uearest pl'ane'oi' separation,

mote new of these"solutions through the cell tend todestroy the planes of separation. The rate of overflow 'is adjusted by sliding'the 'take {ofi "pipes in the sleeves so that more or less pressure willbe required toforce'solution out of the The feed Pi 29 for the common chamber ll preventlarisefin specific gravity which would f communicates with a current density of about 30 amperes per square v source of sodium chromate solution of the desired specific gravity and the supply is regulated to compensate for the rate of consumption of this solution in the cell. Also if the operation of the cell is beingimpaired by accumulation of insoluble impurities and by accumulating caustic soda in volume of soluti'onfed may excess carried off through the overflow pipe 24, this having the effect of flushing the chamber ll of these impurities.

In operating this cell to revivify fouledplated solutions the cell is filled as described and the fouled solution introduced through the pipe 32 into the purified chromic acid layer. As pointed out above the chromic acid content of this purified layer will be increased by operation of the cell and the basic content will be decreased. These actions then purifyv fouled plating acid when it is introduced into the finished chromic acid layer. Since the cell is most conveniently operated by maintaining continuous flow of'solution, a portion of the overflow from the pipe I5 is adjusted in specific gravity and returned-to the cell through the feed pipe 32 along vwith a portion of the fouled acid to be purified. The portion of overflow which is not adjusted and returned is taken as product solution to be used in a chromium plating bath or for the recovery of chromic acid. In a similar manner a portion of the caustic soda solution removed through the overflow pipe 2| may be adjustedand returned through the feed pipe 30 while the remaining portion of 'the overflow may be withdrawn as caustic soda product solution.

Basic substances removed from the floating purified chromic acid layer in chamber I! will be found as an insoluble mud in the feed solution in chamber I4 .of the cell.

By a selection of the proper sizes of cells and current values, an acid making cell as herein described can be operated to treat solution from chromium plating cells so that the emciency of the plating operation is maintained and the chromium content of the plating bath held constant.

In operating the cellit has been found that a foot of area of anode and of cathode chamber section works well.

Referring to. Figure 2 a modification in the location of overflow pipes is seen. In this form the upper overflow pipes i'i' and 2| are located near one end of the cell while the lower overflow pipes l8 and 24' are located near the opposite end of the cell. In other respects-the construction is the same. The location of these overflow pipes, as well as the location of the feed pipes is subject to variation in a horizontal plane, the

only requirement being that the vertical position of their outlets conform with the requirements of solution position.

It'will also be apparent that the cell is not limited to the structure shown with regard to the number of anode and cathode chambers but that any desired number of these upper chambers may be provided communicating with a lower feed chamber. In such a case each anode and cathode chamber is provided with anode or cathode and with the various feed and overflow pipes.

It is obvious that various changes and 'modiflcations may be made in the details of constructhe above specifically deof this invention without tion and design of scribed embodiment departing from the spirit thereof, such changes prises providing a feed solution of an this chamber thebe increased and the chromate solutions.

' tions through the and modifications being restricted only by the scope of the following claims.

What is claimed is:

1. An electrolytic process of chromic acid and caustic for the production alkali which comalkali chromate, floating on said feed solution a less dense product solution of causticalkali, floating on said feed solution independently of said caustic alkali product solution a solution of impure chromic acid less dense than the feed solution, floating on said impurechromic acid solution a less dense chromic acid product solution, and passing current consecutively through .the solutions. I

2. An electrolytic process for revivifying fouled chromic acid plating solutions which consists in continuously providing a feed solution of an alkali chromate, continuously providing a less densecathode solution of caustic alkali floating on said feed solution, continuously providing a crude chromic acid solution less dense than the feed solution and floating on the feed solution out of contact with the cathode solution, conpassing current between the anode and cathode solutions, and continuously drawing off the anode solution purified by operation of the current.

3. In an electrolytic process for the production of chromic acid and caustic alkali from alkali chromates, the steps of, floating a cathode product solution of caustic alkali on an alkali chromate feed solution of greater density, floating out of contact with the cathode solution and also on saiddense feed solution a less. dense impure chromic acid solution, floating on said impure solution a still less dense anode product solution of pure chromic acid, and maintaining the various solutions at their proper volume and density values to retain the various floating relations among them.

4. In an electrolytic process for producing chromicacid and caustic soda, maintaining a feed solution of sodium chromate of specific gravity approximately 1.450, floating thereon a cathode product solution of caustic soda of specific gravity of appro ately 1.200, floating also on said feed solutiog b ut out of contact with thecathode solution an impure chromic acid solution' of specific gravity approximately 1.325, floatingon said impure solution an anode product solution of pure chromic acid of specific gravity approximately 1.200, and passing electric cur- ,rent between the cathode and anode product solutions through the underlying solutions.

5. In an electrolytic process for the production of chromic acid from alkali chromate S0111-1- tions in which the chromic acid product solution is less dense than and is floated on the alkali chromate solution and current passed between them, the step of maintaining a solution of chromic acid of intermediate density floating between the chromic acid product and the alkali 6. In an electrolytic process for the production of chromic acid from alkali chromate solutions wherein independent product solutions of chromic acid and caustic alkali are floated on an alkali chromate feed solution of greater density and current passed between the product solufeed solution; the step which consists in maintaining a solution of chronnc acid of intermegliate density between the chromic acid product solution'an'd the feed solution;

,7. In an electrolytic process for theproduction of 'chromic acid and caustic alkali from alkali ChfOmati'the steps of 'maintaining an alkali' chromate feedfsolution, floating on said feecl solution a caustic alkali cathode product solution of less density, floating also on said feed solution 10 but out of contact with said cathode solution-an intermediate solution "or chromic acid of' less" density, floating on said intermediate 'solutiion' a.

chromic' acid anode product solu'tion of still less density, continuously removing and adjusting each'of said solutions to maintainthe p'roper 5 densities and retiirning the adjusted solutions, and passing electric current between the anode and cathode solutions through the underlying solutions.

' JOHN w. Boss. 10 

